Syntheses of 2,5-Dimethylcyclopentadienyl
<i>ansa</i>-Zirconocene Complexes and Their Reactivity for
Ethylene/Norbornene Copolymerization

C1-bridged and Me<sub>2</sub>Si-bridged 2,5-dimethylcylcopentadienyl <i>ansa</i>-zirconocene complexes,
[Me(H)C(C<sub>5</sub>H<sub>4</sub>)(2,5-Me<sub>2</sub>C<sub>5</sub>H<sub>2</sub>)]ZrCl<sub>2</sub> (<b>6</b>), [(CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>)(H)C(C<sub>5</sub>H<sub>4</sub>)(2,5-Me<sub>2</sub>C<sub>5</sub>H<sub>2</sub>)]ZrCl<sub>2</sub> (<b>9</b>),
[(Me)(H)C(2,5-Me<sub>2</sub>C<sub>5</sub>H<sub>2</sub>)<sub>2</sub>]ZrCl<sub>2</sub> (<b>13</b>), and [Me<sub>2</sub>Si(2,5-Me2C<sub>5</sub>H<sub>2</sub>)<sub>2</sub>]ZrCl<sub>2</sub> (<b>15</b>), are prepared by
using 2-bromo-3-methoxy-2,5-dimethylcyclopentene (<b>7</b>) as a starting material. Solid structures of <b>6</b> and <b>13</b> were determined by X-ray crystallography. The C1-bridged zirconocene
complexes <b>6</b>, <b>9</b>, and <b>13</b> are highly active for ethylene/norbornene copolymerization with high
incorporation of norbornene, while the Me<sub>2</sub>Si-bridged complex <b>15</b> shows relatively low
activity. Complex <b>9</b> shows the highest activity and the highest incorporation of norbornene
among the complexes, but the molecular weight of the obtained polymer is relatively low.